1. Field of the Invention
The field of the invention is printed circuit boards for countering signal distortion and methods of countering signal distortion on a printed circuit board.
2. Description of Related Art
In modern electronic systems, electronic components are typically connected together for communication using printed circuit boards (‘PCB’). A printed circuit board is flat panel that interconnects electronic components using a pattern of flat conductors, often referred to as ‘traces,’ laminated onto a non-conductive substrate. A printed circuit board may contain trace patterns on the top and bottom surfaces of the printed circuit board or in layers through the middle of the printed circuit board. Traces on different layers of a printed circuit board interconnect through ‘vias.’ Vias are conductors that plate the walls of holes extending through the layers of the printed circuit board.
For further explanation, FIG. 1A sets forth a cross-sectional front plan view of an exemplary printed circuit board (100) in the current art that includes a transmission pathway (106) between a transmitter (102) and a receiver (104). FIG. 1B sets forth a top plan view of the exemplary printed circuit board (100) illustrated in FIG. 1A. The transmitter (102) and the receiver (104) of FIGS. 1A and 1B mount to the surface of a printed circuit board and connect at designated portions of the trace pattern called ‘pads,’ or synonymously referred to as ‘lands.’ The transmitter (102) and the receiver (104) may be connected to the printed circuit board using, for example, surface mounting technology, through-hole mounting technology, or any other technology as will occur to those of skill in the art. Surface mount technology connects electronic components to a printed circuit board by soldering electronic component leads or terminals to the top surface of the printed circuit board. Through-hole mount technology connects electronic components to a printed circuit board by inserting component leads through holes in the printed circuit board and then soldering the leads in place on the opposite side of the printed circuit board.
A single printed circuit board typically includes one or more conductive pathways. A conductive pathway is the combination of traces and vias connected together that allow electronic components to propagate signals to one another using electric conduction. The exemplary printed circuit board (100) of FIGS. 1A and 1B includes a transmission pathway (106). The transmission pathway (106) includes trace (110), via (120), trace (112), via (122), and trace (114). The transmission pathway (106) provides a path that allows the transmitter (102) to propagate a signal to the receiver (104) by electrical conduction.
When the signal includes frequency components with corresponding wavelengths comparable to the length of the transmission pathway (106), parasitic elements of the printed circuit board often produce parasitic effects that distort the signal. A parasitic effect is the signal interference that results when the high frequency components of the signal interact with the physical topology of the printed circuit board used to interconnect the components. Examples of parasitic effects include parasitic capacitance and parasitic inductance. Parasitic capacitance is the capacitive interference that results from the storage of stray charges in the PCB elements that make up the conductive pathway between electronic components. Parasitic inductance is the inductive interference that results from the storage of a stray magnetic field around the PCB elements that make up the conductive pathway between electronic components.
A parasitic element is an interconnect component of the printed circuit board that distorts a signal propagated from a transmitter to a receiver. An example of a parasitic element may include an unused portion of via that produces parasitic capacitance when the high frequency components of a signal interact with the via and the reference plane. An example of a parasitic element that produces parasitic inductance may include the conductive trace or via used to connect two components together. As the current oscillates its polarity at high frequencies in these conductors, a magnetic field establishes and collapses around the conductors at a corresponding frequency that operates to resist changes in the current. Often parasitic inductance may be minimized by dedicating an entire layer in the PCB for use by a reference plane. Such a reference plane, however, may operate to produce parasitic capacitance as mentioned above with the unused portion of vias or with mounting pads on the surface of the PCB. Although both parasitic capacitance and parasitic inductance occur from the physical topology of most printed circuit boards, typically one of the parasitic effects—either the parasitic capacitance or the parasitic inductance—will dominate in a particular printed circuit board topology to distort the signal propagated between components.
In the exemplary printed circuit board of FIG. 1A, stub (124) is an unused portion of via (120), and stub (126) is an unused portion of via (122). The stubs (124, 126) are parasitic elements producing parasitic capacitance that distort the signal propagated from the transmitter (102) to the receiver (104). In addition, the traces (110, 112, 114) of FIGS. 1A and 1B may operate as parasitic elements producing parasitic inductance that also distort the signal propagated from the transmitter (102) to the receiver (104).
Current solutions to parasitic effects include traditional techniques such as shorter trace length, back filling, buried vias, and blind vias. Using buried or blind vias to reduce the parasitic effects on a signal, however, increases the cost of designing and manufacturing the printed circuit board. Other current solutions such as designing PCBs with shorter trace length are often impossible to implement because of the physical topology of the printed circuit board. For example, the physical properties of a chip, such as its shape, may not permit shorter trace lengths. As such, readers skilled in the art appreciate that room for improvement exists for countering signal distortion on a printed circuit board.